Heretofore, measuring the rotational envelope intercept position in the well-known cateye oscilloscope display used to produce and utilize customer-engineering and other standard head-alignment disks has been a highly critical operation which was performed manually. The cateye oscilloscope pattern is commonly used to test the alignment of a read-write head directly with a customer-engineering test disk or with a primary-standard test disk. The horizontal displacement of the pattern is proportional to the time relative to a disk-oriented rotational index. The vertical displacement of the pattern is proportional to the instantaneous value of the cateye-signal derived by well-known means. The pattern consists of two intensified sinusoidal curves that are simultaneously displayed 180 degrees out of phase. The three points where the sinusoidal patterns intersect are commonly called rotational-envelope intercepts or simply "intercepts". The sinusoidal enclosed area between any two adjacent intercepts is called a "lobe". The pattern is generated during a single revolution of the disk and is repeated continuously.
To generate the cateye test pattern a special test disk is used with a pair of sinusoidally written simultaneously readable concentric tracks written eccentric to the rotational axis. The track frequencies are typically 1.270 and 1.280 megahertz respectively. The track axis is displaced by a constant distance, here denoted by ".epsilon.", typically 0.0015 inches, from the rotational axis. By knowing the value of .epsilon., the constant magnitude of the radial misalignment ".delta." of the head gap from the nominal track radius can be calculated from the following well-known formula: .delta.=.epsilon. sin (.pi.x.sub.2 /x.sub.1). The angle is in radians, x.sub.1 is the distance between alternate intercepts and x.sub.2 is the linear displacement, positive to the right, of the center intercept from the midpoint between the outer intercepts.
Until now, automated interpretation of the cateye-signal as described above by any other means has been retarded by insufficient knowledge of the signal's analytical nature. The findings as disclosed herein have overcome this restriction and made automation of detecting the intercepts of the cateye-signal possible so the read-write head can be properly and accurated aligned.